The present invention generally pertains to a wafer carrier designed for supporting, constraining, storing and precisely positioning semi-conductor wafer disks for use in the production of integrated circuits. More specifically, the present invention pertains to attaching wafer supports within a wafer container enclosure so as to allow for precise positioning of the wafer supports within the enclosure while also isolating the wafer supports from movement and distortion of the wafer container enclosure walls.
The transformation of semi-conductor wafer disks into integrated circuit chips often involves many steps where the disks are repeatedly processed, stored and transported. The wafers must be transported from workstation to workstation and from facility to facility. Wafer disks are brittle and are easily damaged by physical shock. Also, build-up and discharge of static charges in the vicinity of semiconductor wafers can be catastrophic. Due to the delicate nature of the wafers and their extreme value, it is vital that they are properly protected throughout these procedures from contaminates, and physical and electrical damage.
Specialized carriers or containers are used for handling, storing, and shipping wafers. Such devices normally hold the wafers in axially aligned arrays with, for example, twenty-five wafers in an array. A principal component of the containers is a means for supporting the wafers during handling to protect against physical damage from shock and vibration. This wafer support means may be provided with a path to ground for static dissipation through a machine interface on the bottom of the container. Such containers and support means are known and disclosed, for example, in U.S. Pat. Nos. 5,788,082; 6,010,008; 6,216,874; and 6,267,245, all owned by the owner of this invention, these patents being fully incorporated herein by reference. Such containers are known in the semiconductor processing industry as front opening unified pods or FOUPS.
While these specialized containers have allowed for more efficient automated handling of wafers with less damage, a significant problem has been in precisely locating the wafer disks within the wafer container so as to allow for accurate automated handling of the disks in a production process. Wafer supports and their attaching devices must be designed with sufficient rigidity to firmly and precisely support and locate the wafers when the container is in use. Since the processing of wafer disks is generally automated, it is essential for the container to precisely align the wafer disks according to the specifications of the processing equipment being used. The tolerances available to the container are generally very tight for proper interaction between the processing equipment and the wafer disks. The wafer carrier manufacturing industry constantly strives to design carriers with improved tolerances for better assurance of accurate carrier-equipment alignment.
Generally, wafer containers include machine interface portions, including a guide plate or kinematic coupling and a robotic lifting flange, for moving and properly orienting the carrier with respect to carrier interface portions provided by the processing equipment. Machine interface portions, particularly the kinematic coupling, are often used as a reference point for specifying the relative location of wafer disks for automated processing. The walls of the container, especially the top of the container which is connected to a robotic lifting flange, are susceptible to flexing and distortion when carrying the weight of the container. This flexing and distortional movement is transmitted to the wafer supports through their attachments. Stress and undesired shifting in the position of the wafer supports in the container is the result, increasing the potential for carrier-equipment tolerance mismatch and physical damage to the wafers.
Typically, wafer supports are rigidly attached to the wafer container. Most commonly, threaded fasteners are inserted through an opening in the container wall and threaded into the wafer supports. A problem, however, results from the fact that a small amount of clearance must be allowed between the wafer support and the container walls in order to allow insertion and positioning of the wafer support during assembly of the container. This problem is exacerbated by the variations in dimension and warping of the container enclosure and wafer support sometimes encountered in the injection molding process. These variations make it difficult to manufacture a wafer support with sufficient precision to fit all individual enclosures with a fixed amount of clearance. Often, it is not possible to completely take up the clearance when fastening the wafer support in place, and as a result, the required accuracy for disk positioning is not achieved.
Another problem is presented by differing thermal expansion coefficients of the wafer container shell and the wafer supports. Typically, a wafer container shell is made from polycarbonate material, which generally has a thermal expansion coefficient of about 68xc3x9710xe2x88x926 in./in./Cxc2x0. Wafer supports, however, are often made of PEEK which has a thermal expansion coefficient of about 42xc3x9710xe2x88x926 in./in./Cxc2x0, or of carbon fiber filled polycarbonate or carbon fiber filled PEEK, which have a thermal expansion coefficient of about 20xc3x9710xe2x88x926 in./in./Cxc2x0. Wafer containers are subjected to washing operations, typically at 70-80Cxc2x0. With rigidly attached wafer supports, the much larger thermal expansion coefficient of the shell material with respect to the wafer support material results in stresses in both the wafer supports and the shell. Over time, failure of the components and dimensional creep can result.
Thus, what is needed is a wafer container having wafer supports that provide rigid support and precise positioning of the wafer disks for automated processing. At the same time, the wafer support must be isolated from flexing of portions of the enclosure, and ideally maintain an electrical path to ground. Also, the wafer support should be capable of installation in a container without the need to allow excessive clearance for positioning of the support.
The present invention includes a wafer container having an enclosure portion with an open front for receiving and holding wafers and a door for closing the open front. The enclosure portion has a closed top, a closed bottom, a pair of opposing closed sidewalls, a closed back, and an open front. A pair of wafer supports is positioned inside the enclosure portion, each wafer support having a top margin and a bottom margin and including a plurality of shelves defining wafer seating positions. Each of the wafer supports is rigidly attached to the closed bottom and resiliently attached to the closed top. The resilient attachment of the wafer supports to the closed top may be made by a resiliently flexible plastic connector extending between the closed top and the top margin of each of the wafer supports. Each of these resiliently flexible plastic connectors may have an extension portion that extends from an attachment point at the closed top to an attachment point at the wafer support, and the extension portion may have a greater horizontal component of length than a vertical component. Each of the resiliently flexible plastic connectors may comprise a flat bar with a width generally greater than the thickness and the length generally greater than the width. The wafer supports may be formed of a conductive plastic. The closed top of the enclosure portion may include a robotic flange formed of a conductive plastic and the wafer supports may be resiliently attached to the closed top at the robotic flange by a conductive resiliently flexible plastic spring member.
The present invention may also include a method of constructing a wafer container including the steps of forming an enclosure portion having at least a top and bottom from polycarbonate plastic, molding a pair of wafer supports, positioning the pair of wafer supports in the enclosure so as to define a plurality of wafer seating positions therebetween, rigidly attaching each of the pair of wafer supports to the bottom of the enclosure, and resiliently attaching each of the pair of wafer supports to the top of the enclosure.
The present invention addresses the problems presented by previous wafer support attachment systems in front opening wafer containers. Such containers comprise an enclosure portion with and open front and interior wafer supports positioned at sides of the container portion. A door closes the open front. In the invention, the top of each wafer support is attached with connectors that are oriented so as to at least partially isolate the wafer supports from container shell flexing or movement. The geometry and material of the connectors are selected so as to allow some flexure of the connector, while maintaining enough stiffness so as to prevent excessive movement of the wafer support when fully loaded with wafers.
When the container is in use, the connectors provide stability between the top of the enclosure portion and the wafer supports in the lateral and forward-rearward directions, yet resiliency with respect to the vertical direction. When load is applied to the top of the container, through the robotic lifting flange for example, the resulting deflection of the container housing is taken up in the wafer support connectors. In addition, when the wafer container is subjected to high temperature washing operations, the majority of any differential thermal expansion between the wafer support and the container shell is taken up by the wafer support connectors. In a preferred embodiment, the connectors are configured as cantilevered leaf springs, are integral with the wafer supports, and attach with fasteners to the top of the enclosure portion.
Accordingly, it is an object and advantage of certain embodiments of the invention to provide a rigidly resilient connection at the top of wafer supports to absorb deflection and stress transmitted by loads imposed on a wafer container.
Another object and advantage of certain embodiments of the invention is to provide a connection at the top of wafer supports to minimize the need for wafer support installation clearance to be designed into a wafer container and wafer support thereby improving the dimensional accuracy and process performance of the wafer container.
Another object and advantage of certain embodiments of the invention is to provide a wafer support that has the desirable characteristics of physical compatibility with semi-conductor wafer materials and processing methods.
Another object and advantage of certain embodiments of the invention is to provide a wafer support that has the desirable characteristic of maintaining an electrical path to ground to dissipate static charges.
Another object and advantage of certain embodiments of the invention is to provide a method for wafer container assembly that does not require that installation clearance be pre-designed into an enclosure or wafer supports to allow for positioning of wafer supports.
In a preferred embodiment of the invention, the connectors are molded in one piece with the wafer support. Generally, the material used for the connector will be the same material used for the wafer support itself in order to maintain the desirable characteristics allowed by that material, such as electrical conductivity and physical compatibility with the wafer disk material. Polyetheretherketone (PEEK), particularly with carbon fiber fill, has the desirable characteristics of electrical conductivity and physical compatibility and is the currently preferred material. In a preferred embodiment, two connectors are used to connect each wafer support with the enclosure at the top margin or edge of the wafer support. A rigid method of attachment is used at the opposing margin of the wafer support. The rigid attachment is used at this margin so that a machine interface attached to the bottom of the container can be used as a reference datum to precisely locate the wafer disks during processing. To minimize the effects of tolerance stacking and improve electrical conductivity, portions of the bottom margin of the wafer support preferably extend through apertures in the enclosure bottom and are directly fastened to the kinematic coupling.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.